1. Field of the Invention
The disclosures herein generally relate to a surface laser array and an image forming apparatus, and particularly to a surface emitting laser array that includes plural light emitting units and an image forming apparatus that includes such surface emitting laser array.
2. Description of the Related Art
In a surface emitting laser array that includes plural light emitting units, the polarization directions of plural light beams emitted from the light emitting units are desirably arranged to be in the same direction.
When the polarization directions of the plural light beams emitted from the surface emitting laser array are not uniform, discrepancies in light intensity may occur in the plural light beams that have passed an optical system. For example, when such a surface emitting laser array is used as a write light source of a laser printer, unevenness in color distribution may occur in the output image, for example.
Various techniques have been disclosed for controlling the polarization direction of a light beam emitted from a surface emitting laser to a desired direction (see e.g., Patent Documents 1-3).    [Patent Document 1] Japanese Patent No. 3791193    [Patent Document 2] Japanese Laid-Open Patent Publication No. 2006-013366    [Patent Document 3] Japanese Laid-Open Patent Publication No. 2008-016824
Patent Document 1 discloses a surface emitting laser element having an active layer and a mirror layer arranged on a tilted surface of a semiconductor substrate that is tilted at a predetermined tilt angle with respect to a surface including a crystal axis corresponding to a reference of the semiconductor substrate. Further, an electrode for supplying a current to the active layer via the mirror layer formed above the active layer and wiring for guiding the current to the electrode are formed on the tilted surface. The wiring includes an elongated wiring that is directly connected to the electrode, and the elongated direction of the wiring is arranged to correspond to the tilting direction of the tilted surface. Patent Document 1 also discloses a surface emitting laser array that has the above surface emitting laser elements arranged into a two-dimensional array and a matrix wiring connected to the surface emitting laser elements.
Patent Document 2 discloses a vertical cavity surface emitting laser (VCSEL) that has two orthogonally intersecting planes (X and Y planes) that are perpendicular to a horizontal surface of a substrate and have an origin located around the center of a mesa structure. When the two planes intersect the side surfaces of the mesa structure at plane intersection lines Lx1, Lx2, Ly1, and Ly2 respectively, and when the plane intersection lines Lx1, Lx2, Ly1, and Ly2 make angles of Ax1, Ax2, Ay1, and Ay2 with the horizontal surface of the substrate respectively; at least one of Ax1, Ax2, Ay1, and Ay2 is different from the other.
Patent Document 3 discloses a surface emitting semiconductor laser that has a substrate on which a light emission unit is arranged. The light emission unit has a first multi-layer reflective mirror, an active layer including one or more light emitting regions, and a second multi-layer reflective mirror stacked in this order. At least one of the first multi-layer reflective mirror or the second multi-layer reflective mirror has an oxidized portion distributed unevenly around the region corresponding to the light emitting region. The light emission unit has a groove that surrounds the oxidized portion, and the groove is arranged to have an uneven depth in accordance with the uneven distribution of the oxidized portion.
However, it has been difficult to reduce variations in the polarization directions of plural light beams emitted from plural light emitting units without changing (lowering) the density of the surface emitting laser array.